A machine for automatically inflating air cushions. The cushions have two plastic walls (21; 22) joined together around their periphery to define a closed space. The machine comprises a source of gas under pressure, and two jaws (1; 2) arranged such that they can move on a supporting structure. Each one of the jaws consists of a body (1; 3) each having a plane face (4:10) of substantially the same shape and surface area, it being possible for the faces to mate against one another in order to clamp between them the walls (21; 22) of a cushion. Each jaw comprises an aspiration channel (6; 7) opening onto its planer face. A first jaw on the inside of its body has a hollow needle (5) able to move relative to the body and capable in a certain position of having its pointed tip outside of the surface of the plane face. A second jaw within its body has a heating element (18) capable, when the two jaws are clamping between them the two walls of a cushion, of heat-welding the two walls together over a certain proportion of their contacting surface.